1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an inter-stage seal in a gas turbine engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, a turbine converts the energy from a hot gas flow into mechanical energy used to drive the compressor and, in the case of an industrial gas turbine (IGT), to drive an electric generator for power production. A typical IGT turbine includes four stages of stator vanes and rotor blades to progressively extract the energy from the hot gas flow.
In the multiple stage turbine with rotating blades and stationary vanes or stators, inter-stage seals are used on the inner diameter of the stator to form a seal between the rotating blades and the stationary vanes. The seal is exposed to a pressure differential which is identical to that created by the acceleration of the flow between stator vanes. Flow which leaks across this seal affects the performance of the engine in several ways. First, the leakage affects the aerodynamic design of the turbine and also makes it difficult to control the turbine rim cavity purge using expensive compressor bleed air to the minimum required to avoid hot gas ingestion. Large inter-stage seal clearance ultimately leads to over purged turbine cavities which further reduce engine performance by not being able to extract work from the compressor bleed air. A well designed system minimizes the leakage across the inter-stage seal to satisfy rim cavity purge and rotor cooling requirements. While many newer types of contacting and non-contacting seals exist to run tight at steady state conditions, the concern is seal wear during transient operation.
The rotor and stator systems are not perfectly thermally matched so the transient thermal response of the stator does not match that of the rotor. This will cause either wear on seals that are assembled tight, or on seals which are not allowed to contact, will require the steady state clearance at base load to be opened due to the transient close down which are most severe during warm restarts.
An ideal solution to this problem is to have the rotor and stator thermally matched so the stator thermally grows identical to the rotor and the clearance is a function of mechanically induced displacement. Cold or assembly clearances can be built identical to the mechanical growth of the rotor. This would make the seal clearance effectively zero or line to line and would offer the particular seal its lowest flow consumption. This type of system would be classified as passive clearance control. In particular, the rotors are usually large compared to stators and would require adding mass to the stator or changing the external environment of the casing. In aircraft engines, this approach is prohibitive due to weight constraints. Other approaches to passive clearance control would be to use seal support materials which have a low coefficient of thermal expansion in combination with spring like stator designs to absorb the relative motion between the stator and the lesser moving seal support.
The prior art U.S. Pat. No. 6,761,529 B2 issued to Soechting et al on Jul. 13, 2004 and entitled COOLING STRUCTURE OF STATIONARY BLADE, AND GAS TURBINE discloses an inter-stage seal in a turbine. However, this seal is not controlled. The seal gap between the rotor and the stator only changes due to thermal mismatches.